Systems and methods for creating variable optical effects on media

Abstract

 Methods and systems for creating optical effects in pigments, inks, and on media are described. There is in particular described a system or indicia for use as a security or identification feature comprising a combination of material that is capable of inducing a change of optical appearance of the system or indicia in response to exposure to ultraviolet (UV) light. The combination of materials includes at least a first material that changes its optical appearance under exposure to UV light and a second material that provides a color under ambient visible light or under exposure to UV light. The first and second materials are selected in such a way that exposure to UV light causes a visible spectrum of the system or indicia to dynamically shift over time from a first spectrum to a second spectrum distinct from the first spectrum

Description

BACKGROUND OF THE INVENTION

[0001] This application is a division of European patent application 06773957.3 of June 22nd , 2006 which derives from PCT application PCT/US2006/024721.

[0002] At least certain embodiments of the present inventions relate to pigments, inks, and other materials which may be applied onto media, such as paper, or other materials. These systems and methods may be used as security features or identification features to allow an object to be authenticated.

[0003] Existing methods that use UV (ultraviolet) light to activate a visible effect produce only a static emission spectrum. Such features rely on fluorescence in which UV light is absorbed by a material and then re-radiated as a longer wavelength, typically in the visible spectrum. The emitted light is determined by the chemical and electronic structure of the compound and is a material property thereof. The spectrum of the light thus induced does not change under steady-state illumination or under intensity-varying conditions.

[0004] Existing methods also include the use of a photochromic material in authentication systems, such as security printing inks. See, for example, Organic Photochromic and Thermochromic Compounds, Volume 1, edited by John C. Crano and Robert J. Gugoielmetti, 1999 (ISBN 0-306-45882-9).

SUMMARY

[0005] Various embodiments of systems and methods for inducing a dynamic, reversible color-changing property are described in this disclosure. This summary sets forth certain embodiments but not necessarily all embodiments which are described herein.

[0006] In one embodiment, a pigment particle includes a core having a carrier substance and a fluorescent material (or a phosphorescent material), wherein the core has a substantially spherical shape, and the pigment particle also includes a shell which surrounds the core, and the shell includes a photochromic material which has a first optical property when illuminated by a first light source and a second optical property when illuminated by a second light source, which includes a set of wavelengths not sufficiently present in the first light source, and wherein the second optical property attenuates an emitted radiation from the fluorescent material. In at least certain embodiments, the first optical property is substantially transparent and the second light source includes ultraviolet wavelengths which are the set of wavelengths not sufficiently present in the first light source. The UV excitation from the UV in the second light source initially causes the pigment particle to have a first appearance based on the fluorescent emissions (e.g. the fluorescent emissions at least in part effect the first appearance by having an effect of the color of the first appearance). Continued UV excitation causes the pigment particle to have a second appearance based on the second optical property of the photochromic material which attenuates an emitted radiation from the fluorescent material. Typically, the first appearance is a first color based in part upon the fluorescent emissions from the fluorescent chromophore (a fluorescent material) and the second appearance is a second color which is different than the first color. Also typically, the photochromic material changes from the first optical property to the second optical property, while under UV illumination from the second light source, over a period of time of more than about one-third of a second to as long as about 30 seconds. This pigment particle may be mixed with an ink, and the ink can be used to print onto currency or other objects to create an indicia or identification feature on the object. The identification feature can be authenticated by illuminating the indicia with a UV light source to thereby cause the color change, under the continued UV illumination, from the first color to the second color. Thus, the pigment particle may be used in currency to authenticate the currency or on other objects to authenticate or identify the object. Further, this pigment particle can also be used to produce decorative or attention-grabbing features, for instance, on products or packaging.

[0007] In another exemplary embodiment, an object includes a substrate and an indicia which includes a first material and a second material. The first material is coupled to the substrate and the second material is coupled to the substrate. The first material has a first optical appearance before UV excitation and a second optical appearance after receiving UV excitation. In at least certain embodiments, the first material is a photochromic substance which is transparent under ambient light which has insufficient amounts of ultraviolet light and darkens or reflects a color in the presence of UV excitation. In this case, the first optical appearance is transparency and the second optical appearance is a darkened or colored appearance after receiving UV excitation. In other embodiments, a saturable absorber may be used as the first material rather than a photochromic material or substance. The second material may be a fluorescent chromophore or a non-fluorescent pigment which is capable of providing the color. The indicia appears to radiate a first color upon initial UV excitation while the first material has the first optical appearance, and then over a short period of time, appears to radiate a second color upon continued UV excitation as the first material has changed to the second optical appearance. The first material typically attenuates the fluorescent radiation from the fluorescent chromophore, and the first color is based at least in part on the fluorescent radiation from the fluorescent chromophore (e.g. at least a portion of the spectrum of this first color is dependent upon the fluorescent chromophore). The second color may be based upon reflections from the first material in the second optical appearance, or may be based upon the fluorescent radiation from a third material which is a fluorescent chromophore. The photochromic material typically changes from the first optical appearance to the second optical appearance over a short period of time, such as a period of more than about a third of a second to a period of less than about 30 seconds, such that the first color is visible for at least about a third of a second to as long as 30 seconds. The object may be currency and the indicia created by the first and second material may be an authentication or an identification feature placed on the currency through a printing process or through other methods of applying the material onto a media.

[0008] In yet another embodiment of the inventions described herein, the object includes a substrate and an indicia disposed on the substrate, wherein the indicia includes a first material and a second material, each of which are coupled to the substrate. The first material has a first optical appearance when illuminated by a light source which does not include a sufficient amount of UV wavelengths and a second optical appearance after receiving sufficient UV excitation from a light source containing a sufficient amount of UV wavelengths. The indicia appears to radiate a first color at a first intensity upon initial UV excitation while the first material has the first optical appearance (e.g. the first optical appearance is transparent for a photochromic material) and then appears to radiate the first color at a second intensity upon continued UV excitation as the first material has changed to the second optical appearance. In at least certain embodiments, the second material may include a fluorescent chromophore which emits fluorescent radiation in response to UV illumination, and the first material includes a photochromic material or substance which, when having the second optical appearance, attenuates the fluorescent radiation from the fluorescent chromophore and wherein the first color is based upon the fluorescent radiation from the fluorescent chromophore (e.g. at least a portion of the spectrum of the first color is derived or based upon the fluorescent emission from the fluorescent chromophore).

[0009] Various methods for authenticating or identifying objects by using an indicia or features on the object are also described herein. In one exemplary method, a material on the object is exposed to a set of wavelengths, and the user observes radiation or emission of a first color in response to the exposing, and the user observes a radiation of a second color, also in response to the exposing and after the observing of the radiation of the first color. By observing both the first and second color while under the same illumination source, such as an ultraviolet light source, and by observing the change over a short period of time, the user can determine the authenticity of the object if it displays the anticipated color change from the first color to the second color in a short period of time. The material may have an initial color prior to the exposing, and the material may appear to change from this initial color, then to the first color, and then to the second color in response to the exposing and, after the exposing has stopped, the material reversibly returns to the initial color. The time between changing from the first color to the second color is typically more than about one-third of a second to less than 30 seconds, such that the first color is visible for at least about a third of a second to as long as 30 seconds. Typically, at least in certain embodiments, the material may be a photochromic material and may also include a fluorescent material.

[0010] Another exemplary method to authenticate an object includes exposing a material with an illumination at a first intensity and emitting, in response to the illumination at the first intensity, a first color, and exposing the material with an illumination, such as an ultraviolet illumination, at a second intensity and emitting, in response to the illumination at the second intensity, a second color. Typically, the illumination is of the same frequency but different intensities, and the material includes a first fluorescent material having a first efficiency and a second fluorescent material having a second efficiency which is significantly less than the first efficiency. The imbalance in the efficiencies causes the fluorescent chromophore which has the much higher efficiency to dominate the resulting emissions spectrum when the illumination source is at a lower intensity. When the illumination source is at a higher intensity, then both fluorescent chromophores can effect the resulting fluorescent spectrum, and the color will appear slightly different than the resulting color at the lower illumination intensity.

[0011] Other exemplary embodiments include an object which comprises a substrate, a pigment coupled to the substrate, an absorption varying material, such as a photochromic substance, coupled to the pigment and disposed over the pigment, and a fluorescent material, such as a fluorescent chromophore, coupled to the absorption varying material and disposed over the absorption varying material. The absorption varying material changes its optical property in the presence of ultraviolet or other types of illumination to cause a change in the appearance of the combination of the pigment, the absorption varying material, and the UV fluorescent material, which together may form an indicia on the object.

[0012] In yet another exemplary embodiment, an object includes a substrate and an indicia which includes a fluorescent material and a phosphorescent material. The fluorescent material emits a fluorescent radiation when under illumination from a radiation source, such as an ultraviolet light source, and the phosphorescent material phosphoresces a phosphorescent radiation when under illumination from the radiation source (e.g. a UV light source) and continues to phosphoresce for a period of time after the illumination has stopped.

[0013] Other objects, indicia, pigment particles, and methods for using these objects are described further below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present inventions are illustrated by way of example and not limitation in the figures of the accompanying drawings. The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.

[0015] Figure 1 shows an exemplary object displaying a color shift in response to a change of illumination.

[0016] Figure 2 shows an object that displays a color shift in response to continued illumination from a light source, such as an ultraviolet light source.

[0017] Figure 3 shows another exemplary object that displays a color shift in response to a change of illumination (e.g. a change from ambient light which has an insufficient amount of ultraviolet radiation to illumination under an ultraviolet light source).

[0018] Figure 4 shows an exemplary object that displays a color shift in response to continued illumination from a light source, such as an ultraviolet light source.

[0019] Figure 5 shows an exemplary object that displays a color shift in response to a change of illumination.

[0020] Figure 6 shows an exemplary object that displays a color shift in response to continued illumination from a light source, such as an ultraviolet light source.

[0021] Figure 7 shows another exemplary object that displays a color shift in response to a change of illumination.

[0022] Figure 8 shows another exemplary object, which may be a pigment particle, that displays a color shift in response to a change of illumination.

[0023] Figure 9 shows another exemplary object, such as a pigment particle, that displays a color shift in response to a continued illumination from a light source, such as an ultraviolet light source.

[0024] Figure 10 shows another exemplary object, such as a pigment particle, that displays a color shift in response to continued illumination from a light source.

[0025] Figure 11 shows another exemplary object that displays a color shift in response to continued illumination from a light source.

[0026] Figure 12 shows another exemplary object that displays a color shift in response to continued illumination from a light source.

[0027] Figure 13 illustrates pigment particles which include fluorescent material and phosphorescent materials which exhibit one color under initial UV excitation and another color after the UV excitation is removed, and the red or other color phosphorescence continues after the UV excitation is removed.

[0028] Figure 14 illustrates another exemplary object which includes phosphorescent and fluorescent materials.

[0029] Figure 15 shows another exemplary object, such as a pigment particle, that includes both fluorescent material and phosphorescent material which may be used in indicia of the present invention.

[0030] Figure 16 shows another exemplary object, such as pigment particles, which may be used in indicia according to the methods and systems of the present invention.

DETAILED DESCRIPTION

[0031] The subject inventions will be described with reference to numerous details set forth below, and the accompanying drawings will illustrate the inventions. The following description and drawings are illustrative of the inventions and are not to be construed as limiting the inventions. Numerous specific details are described to provide a thorough understanding of the present inventions. However, in certain instances, well known or conventional details are not described in order to not unnecessarily obscure the present inventions in detail.

[0032] This disclosure describes methods and techniques for creating dynamic color-changing effects on substrate material that can be induced by exposure to, for example, ultraviolet radiation. A substrate thus prepared can be readily differentiated from ones that contain only reflective regions or UV fluorescent regions. Such effects can be used to produce security and identification features on currency, branded products, documents, and other substrate material. They can also be used to produce decorative or attention-grabbing features on, for instance, products and packaging.

[0033] At least certain embodiments described herein provide a means of using UV light, under either steady -state conditions or intensity varying conditions, to excite visible light (and IR) with a color spectrum that changes quickly with time (e.g. in as short as about one-third of a second to as long as about 30 seconds). Such features can be used to provide a greatly enhanced overt feature for currency and other secure documents and branded products. For example, such features applied to currency may allow for the detection of counterfeit currency.

[0034] The pigments, inks/coatings, and printing/coating processes that make up the color-changing features of at least certain embodiments of these inventions provide unique ways to enable a person with a low-cost radiation source, such as a UV light, to interrogate a secure document such as a bank note or currency. Unlike existing UV induced features, which merely fluoresce with a constant color spectrum under UV illumination, the color changing embodiments described herein provide a dynamic color changing effect that is very unique and not currently available on any document. When these features are integrated onto substrate material via printing or coating with an ink (varnish, etc.) using one (or a combination) of the described methods, and subsequently exposed to UV light, they will provide an unmistakable overt or semi-covert feature where applied. (Various methods for integrating the features into a substrate, beyond printing, such as injection molding, casting, painting, dipping, etc. can also be used to produce the feature and as such, the embodiments are not limited merely to printing applications.)

[0035] The fundamental properties of the materials to be integrated onto substrates and used for these embodiments may include four fundamental types. The first is fluorescent materials, which emit visible and/or IR light upon absorption of UV light. The second is photochromic materials, which darken or otherwise change their absorption spectrum in response to UV. Both positive and negative photochromic materials may be used. In certain embodiments, thermochromic substances may be used in place of or in addition to the photochromic substances. The third is saturable absorbers, which initially absorb but become transparent as the transitions become saturated. The fourth is materials that undergo an internal energy transfer mechanism in the presence of UV as a function of intensity. Intensity, defined as energy per unit time per unit area, provides the control variables for effecting the fourth change.

[0036] Materials with the above mentioned properties can be synthesized into pigments, small particles, coatings, sprays, inks, or other media, which can be applied to substrates to create objects with indicia which may be used to authenticate or identify the object. Application processes include mixing small particles of materials with these properties into inks and coatings, and printing them onto surfaces. Multiple materials can be mixed together to provide a coating with combined features. Additionally, multiple layers can by applied on top of one another to provide dynamic responses to UV excitation. Additionally, coated regions having different responses to UV can be applied in close proximity to provide an even larger array of dynamic responses. Photochromic materials are described in an article titled "Organic Photochromism" by H. Bouas-Laurent and Heinz Durr, Pure Applied Chem., 73:4, pp. 639-665 (2001); this article is incorporated herein by reference.

Categorization of Color-Changing Effects

[0037] The concepts disclosed in the following figures (Figures 1-16) for integrating UV-induced color changing effects into pigments and media can be broken down systematically into the following categories describing the functional exposure method for how the color changing mechanism is induced:

a.) Color change occurs upon moving the substrate from ambient (visible room) light to UV light. (Exposure spectrum changes to induce the effect)

b.) Color change occurs under extended (typically about a few seconds, such as about 3 seconds to 7 seconds) exposure to UV light. (Effect is typically induced under constant exposure spectrum)

c.) Color change occurs upon moving substrate from UV to ambient light or from UV to no light. (Exposure spectrum changes to induce the effect)

d.) Color change occurs under different intensities of UV exposure (focused vs. nonfocused). (Intensity of exposure changes, but not spectrum of exposure, to induce the effect)

[0038] These categories can be further broken down into monolithic vs. mixed pigment types, and even further into single layer vs. multi-layer application requirements.

1. Monolithic pigments (a single pigment exhibits all of the effects)

i. in conjunction with UV induced photochromic/saturable absorbers

ii. in conjunction with UV induced photochromic/saturable reflectors

iii. multiple emission/reflection wavelengths

iv. multiple emission lifetimes

v. involving intensity dependent cross sections and/or energy transfer

vi. single emission/reflection wavelengths

2. Mixed pigments (multiple pigments are used to achieve the effect)

i. in conjunction with UV induced photochromic/saturable absorbers

ii. in conjunction with UV induced photochromic/saturable reflectors

iii. multiple emission/reflection wavelengths

iv. multiple emission lifetimes

v. involving intensity dependent cross sections and/or energy transfer

vi. single emission/reflection wavelengths

[0039] The following charts categorically detail these various concepts for integrating color-changing effects into pigments and printed/coated media. Each concept is shown in the corresponding figure which is labeled with the same number; for example, concept 16 is shown in Figure 16. Combinations of the effects can be used to provide additional color-changing performance.
a. Change occurs upon moving pigments/substrate from ambient room light to UV light Pigment Type

Concept	Pigment Type						Print Layers		Performance
	Monolithic			Mixed
	florescent	reflective	fluor/refl	florescent	reflective	fluor/refl	Single	Multiple	I	ii	iii	iv	v	vi
1						x		x	x		x
3						x		x		x	x
5					x			x		x	x
7					x			x	x
8			x				x			x	x

b. Color change occurs upon extended (few seconds) exposure to UV light.

Concept #	Pigment Type						Print Layers		Peformance
	Monolithic			Mixed
	fluorescent	reflective	fluor/refl	fluorescent	reflective	fluor/refl	Single	Multiple	I	ii	iii	iv	v	vi
2				x				x	x		x
4						x		x		x	x
6						x		x		x	x
9			x				x		x		x			x
10	x						x		x		x
11						x	x		x					x
12						x	x				x

c. Color change occurs upon moving pigment/ substrate from UV to ambient light or from UV toto no light.

Concept #	Pigment Type						Print Layers		Performance
	Monolithic :			Mixed
	florescent	reflective	fluor/refl	florescent	reflective	fluor/refl x	Single	Multiple	I	ii	iii	iv	v	vi
13						x	x				x
14						x		x			x	x
15	x						x				x	x

d.Color change occurs upon exp[osure to varying intensities of UV light, i.e, focused VS nonfocused.

Concept #	Pigment Type						Print Layers		Performance
	Monolithic			Type Mixed
	fluorescent	reflective	fluor/refl	fluorescent	reflective	fluor/refl	Single	Multiple	I	ii	iii	iv	v	vi
16	x						x				x		x

[0040] In addition to undergoing the described change in appearance, it may be necessary or desirable in some instances for the feature to revert to the original (or closely thereto) condition that it existed in prior to UV exposure. Additionally, it may be desirable for the feature to undergo the cycle of changing under UV and returning closely to its original condition a large number of times. In order for this to happen, the fluorescent materials should emit their absorbed energy and return to their unexcited states. The photochromics and saturable absorbers must also lose any stored or trapped energy and return close enough to their original states so that the process can be cycled. The ability to undergo multiple numbers of cycles of the feature is thereby a component of exemplary embodiments of at least some of these inventions.

[0041] In other instances, it may be desirable for the feature to remain, at least partially, in its altered state. Photochromics with very stable open forms can provide very long-lived features for such a performance requirement. Thus, permanently altered or long-lived alterations to the color spectrum of the feature upon UV excitation is a further aspect of at least certain embodiments of the inventions. In many of the embodiments described herein, a phosphorescent material may be used in place of a fluorescent material. The primary difference between these materials is that the phosphorescent material continues to emit radiation for a short period of time after the excitation illumination (e.g. UV) has been stopped.

[0042] The embodiment of Figure 1 uses multiple layers of material to achieve the desired effect. In this embodiment a reflective pigment (shown red) would be applied to a substrate (e.g. paper for a currency or another object), followed by the application of an absorption varying material (photochromic or saturable absorber), followed by a UV fluorescent pigment (shown green). Under ambient visible light, the photochromic material is transparent, and the UV fluorescer is colorless, so the substrate material appears red to the observer. When exposed to UV light, the fluorescer begins to emit green light, which in conjunction with the reflected red, changes the appearance of the substrate to a color representing a mixture of these wavelengths (red + green). As the photochromic begins to darken, light is blocked from the red reflective pigment, and the observed color that is delivered to the observer from the substrate contains only the fluorescent emission (i.e., green). Thus, the original red color of the substrate undergoes a color shift (from red to yellow to green in this example) when the substrate is exposed to UV radiation. Though red and green have been used as representative colors in this example, clearly other wavelengths could be used, including IR components which would provide the effect for machine-reading interrogation systems.

[0043] The use of a saturable absorber in place of the photochromic would effectively cause the intermediate layer between the colors to bleach from opaque to transparent yielding an exposure induced color spectrum that combined the fluorescent emission (shown green) with the reflected spectrum (shown red), rather than only the fluorescent color. In this case, the emission presented to the observer would start out with the reflected color of the saturable absorber, then turn to green (fluorescence) then to yellow as the absorber faded and red could show through. Though the following concepts will be described in terms of photochromics, the effects can also be induced with the use of saturable absorbers used in place thereof.

[0044] It should be noted that in addition to visible emission lines, each of these embodiments can incorporate non-visible emission lines (UV and IR) that can be detected with appropriate photodetectors, thereby providing additional functionality for the material as a security feature.

[0045] The embodiment of Figure 2 uses multiple layers of material to achieve the desired effect. A fluorescent layer (shown in red in Figure 2) is applied to the substrate first, followed by a photochromic (or saturable absorber) layer, followed by another fluorescent layer (shown as green in Figure 2). Under ambient room light conditions, both the fluorescent pigments and the photochromic layer will nominally be colorless, possibly even enabling a reflective layer to show through from beneath the red fluorescent layer. The initial color spectrum presented to the observer upon exposure to UV will thus be a mixture of fluorescence from both the red fluorescent layer and the green fluorescent layer (red + green = yellow). As the photochromic layer darkens, and begins to reduce both the UV excitation from the red under layer and its emission, the observed color spectrum will shift from yellow (mixed) to green (top layer).

[0046] If a saturable absorber is used instead of a photochromic, then the spectrum will shift in the reverse order, namely from fluorescent green to fluorescent yellow as the red fluorescence is enabled.

[0047] This embodiment has advantages over the one described in connection with Figure 1 in that it can be induced entirely with UV light in the absence of any visible light.

[0048] The embodiment of Figure 3 uses multiple layers of material to achieve the desired effect. A reflective layer (shown red) is applied first to the substrate, followed by a photochromic (or saturable absorber) layer that undergoes a change from transparent (a first optical property or appearance) to a color (a second optical property or appearance) other than gray or brown (shown blue), followed by a fluorescent layer (shown green). Under ambient illumination the substrate appears red to the observer. When exposed to UV, the spectrum emitted from the substrate will initially be a combination of reflected red and fluorescent green (yellow). As the photochromic a material takes on its characteristic blue color, and suppresses the red, the substrate spectrum as presented to the viewer will consist of components from the fluorescent layer (green) and the reflected photochromic layer (blue). Thus it will appear, at least for this example, to shift from yellow to aqua in appearance.

[0049] If a saturable absorber is used instead of a photochromic material, and a transition from reflected blue to transparent occurs in this layer, then the color spectrum will shift from aqua (fluorescent green + reflected blue) to yellow (fluorescent green + reflected red).

[0050] The embodiment of Figure 4 uses multiple layers of material to achieve the desired effect. The first layer contains a fluorescent material (shown red), and the second layer contains a photochromic layer that changes from transparent to blue. Both layers are colorless under ambient illumination, possibly allowing a reflective under-layer to show through. Upon exposure to UV, the initial spectrum emitted from the substrate will appear red (from a red fluorescent emission). As the photochromic layer begins to turn blue, it will suppress the red fluorescence and the color spectrum emitted by the substrate will shift first to purple (red + blue) then to blue.

[0051] As with the previous embodiments, a saturable absorber or reverse-saturable absorber can be used in place of or in addition to a photochromic to provide a different shift in the color spectrum. Combining features thusly would enable for instance, two (or more) features in the same vicinity on a substrate to undergo opposite color shifts simultaneously. An example might include a red to green shifting region next to a green to red shifting region on the same object.

[0052] The embodiment of Figure 5 uses multiple layers of material to achieve the desired effect. A reflective layer (red) is applied to the substrate, followed by a photochromic (or saturable/reverse saturable absorber) layer that changes to a color (shown blue) under UV excitation. Ambient light that reflects off of the surface of the feature will provide red emission to the observer. Upon exposure to UV, the red color will shift to blue as the photochromic material darkens and begins to reflect blue from both the UV lamp and from the room.

[0053] The embodiment of Figure 6 uses multiple layers of material to achieve the desired effect. A fluorescent layer (shown green) is applied to the substrate, followed by a photochromic (saturable/reverse saturable absorber) layer that changes to a color (shown blue) upon excitement with UV. Both layers are fairly colorless and featureless under ambient light, perhaps showing a reflective pigment below the fluorescent pigment layer. When excited with UV, the fluorescent layer begins to absorb and emit light, providing a color spectrum to the observer that is largely green in the example of Figure 6. As the photochromic layer begins to darken and turn blue, the color spectrum shifts to that of reflected blue in the example of Figure 6.

[0054] An additional manner in which this feature can be used is to filter out a part of the fluorescent spectrum so that other portions of it continue to emit. Appropriate control of the concentration of the photochromic pigment in the carrier can provide for some amount of continuous through put of UV to the underlying fluorescent layer, allowing it to continuously absorb and emit light. The UV induced colored photochromic layer can be selected to absorb only a portion of this spectrum, passing the remainder. In this manner, fluorescence from the under layer can always constitute a portion of the dynamic spectrum, both before and after the photochromic layer changes between its optical properties.

[0055] The embodiment of Figure 7 uses multiple layers of material to achieve the desired effect. Under ambient light the substrate emits a reflected (shown red) spectrum to the observer. Under UV excitation, the photochromic layer darkens and the reflected red spectrum diminishes in intensity. The color does not change spectrally but the brightness of the color does.

[0056] The embodiment of Figure 8 uses multiple layers of material integrated into individual particles to achieve the desired effect. By integrating some or all of the overall layered performance into a heterogeneous particle, instead of separating each feature into different particles, the requirement to print/apply multiple layers of material can be reduced, thereby simplifying the application process.

[0057] This pigment contains an inner reflective (shown red) core, coated with a photochromic material that will (reversibly) change from transparent to colored (shown blue for particle 82 or green for particle 84) when exposed to ultraviolet radiation of the appropriate wavelength and intensity. The particles are applied to the substrate in sufficient manner and quantity to achieve the desired effect under the requisite exposure conditions. The particle 80 shows the appearance of the particle in ambient room lighting (e.g. conventional incandescent bulbs or fluorescent tubes) which lacks a sufficient amount of UV radiation or wavelengths to cause fluorescence or to cause darkening of the photochromic material.

[0058] The embodiment of Figure 9 integrates multiple spectral functionalities into a layered particle system. This particle consists of a fluorescent inner core (shown yellow in particle 90), coated with a photochromic layer that will change to a color (shown blue in particle 92) or darken (shown gray in particle 94). The core may have a substantially spherical shape, and the photochromic layer may be a shell which surrounds the core. Under normal ambient lighting conditions the particle is relatively colorless, enabling it to be non-apparent to an observer. Its colorless properties in ambient light also enable it to be coated over reflective regions, which are intended to be colored for some designated purpose. When exposed to UV radiation, the colorless pigments begin to fluoresce (yellow) as in particle 90 and the photochromics then begin to darken or change color to produce the resulting particle 92 or 94.

[0059] The photochromics that change color can act as a filter to the fluorescent light, absorbing some of it and transmitting other frequency bands so as to cause the initial (yellow) spectrum to shift to green (as in the case of particle 92) as the red component of the multi-component spectrum is absorbed and the green is transmitted. Photochromics that darken in more of a neutral density (broad spectrum) manner, rather than filtering out specific bands, can be used to reduce the overall brightness of the pigment. Thus the effect would be to reduce the brightness of the original color rather than to change it by very much (which is shown as the case of particle 92).

[0060] The core may be formed using any one of a variety of methods for forming a carrier substance with a pigment such as a fluorescent chromophore. Examples of such methods are described in the following articles: "Monodisperse Colloidal Silica Spheres from Tetraalkoxysilanes: Partial Formation and Growth Mechanism," A. Van Blaaderen, J. Van Geest, and A. Vrij, Journal of Colloid and Interface Science, 154:2, (December 1992); and "Synthesis and Characterization of Colloidal Dispersions of Fluorescent, Monodisperse Silica Spheres," A. Van Blaaderen and A. Vrij, Langmuir, 8:12, (1992). The shell may be formed using any one of a variety of methods for depositing or precipitating the materials of the shell onto the core. Examples of shell formation methods are described in: "Photochromic Behaviour of a Spirobenzopyran chemisorbed on a Colloidal Silica Surface," M. Ueda, K. Kudo and K. Ichimura, J. Mater. Chem., 5:7, pp. 1007-1011 (1995); and "Luminescence Lifetime Temperature Sensing Based on Sol-Gels and Poly(acrylonitrilc)s Dyed with Ruthenium Metal-Ligand Complexes," G. Liebsch, 1. Klimant, and O.S. Wolfbeis, Advanced Materials, 11:15, (1999).

[0061] The embodiment of Figure 10 integrates multiple spectral functionalities into a layered particle system. The particle contains an inner fluorescent core (shown red), covered with a photochromic (saturable/reverse saturable absorber) layer, covered with a second fluorescent layer (shown green). The core may have a substantially spherical shape, and the photochromic layer may be a shell which surrounds the core, and the fluorescent layer may be another shell which surrounds the first shell. Under typical ambient illumination, the particle could be relatively colorless, enabling it to be readily integrated onto a substrate with other reflective features. Upon exposure to UV radiation, both fluorescent layers would begin to absorb UV and emit their respective colors, creating in this case yellow. Upon exposure to UV, the photochromic layer will darken, blocking some of both the excitation light and the fluorescence from the core. A shift from yellow (mixed red and green fluorescence) to green would thereby be the gradual result of an applied region of such particles thus exposed to UV.

[0062] The embodiment of Figure 11 integrates some or all of the dynamic color-changing functionality into a mixed particle system. The system contains two (or more) separate fundamental particles. One group of which imparts reflective or fluorescent functionality (red) to the mixture, and one that imparts photochromic functionality to the mixture. When viewed under ambient lighting conditions, the mixed particle system reflects the color spectrum of the reflective particles (red). When exposed to UV, any fluorescent particles in the mixture will begin to fluoresce, and the photochromic particles will begin to darken and reduce the light available to both reflect off of the reflective pigment particles and to excite the fluorescence. Thus, UV exposure causes the reflected and/or fluorescent color spectrum to diminish in brightness.

[0063] Saturable and reverse-saturable (s/rs) absorbers could also be integrated into the mixed particle system causing it to shift from a darker color to a reflected/fluorescent color as the transitions saturated and the s/rs absorbers became transparent.

[0064] The embodiment of Figure 12 integrates some or all of the dynamic color-changing functionality into a mixed particle system. The system uses one or more fluorescent particles mixed, in an appropriate ratio, with photochromic (s/rs absorber) particles which are separate from the fluorescent particles. The photochromic changes to a color (shown blue) upon exposure to UV, creating a filter for some of the fluorescent emission lines. Under typical ambient illumination, this system could be close to colorless enabling it to be integrated into reflective features on a substrate without being highly noticeable.

[0065] Upon excitation with UV, the fluorescent particles will begin to emit light at their characteristic wavelengths. As the photochromics begin to change their absorption properties, typically with a slightly longer response time on the order of a few seconds, they will begin to block UV from the fluorescent particles somewhat, and to filter out some of the emitted fluorescence. This change in the absorption of the photochromic particles will thereby alter the spectral constituents of the light that is delivered to the observer from the fluorescers. In addition, the photochromic particles will contribute an altered reflectivity (shown blue) to the observed color spectrum delivered to the observer.

[0066] As with the other embodiments, it is possible to integrate non-visible emission lines into the light-emitting particles (fluorescers) to increase their utility as security pigments, as such invisible lines can generally only be detected with electronic means (photodetectors), and not the eye.

[0067] The embodiment of Figure 13 integrates some or all of the dynamic color-changing functionality into a mixed particle system. The system integrates two or more pigments that fluoresce under UV but which have different time-constants for their emission, such as a fluorescent material (as one pigment) and a phosphorescent material (as another pigment). Under typical ambient lighting conditions, these particles can have little or no observable color rendering them appropriate for over laminates (or coatings) on reflective inks, coatings, and substrates. When exposed to UV, the particles begin to absorb light and subsequently re-emit the light in their respective wavelength bands. Upon cessation of the UV exposure, the fluorescent emission diminishes rapidly, on the order of nanoseconds, but the phosphorescent materials continue to emit for a while. Thus, a substrate that initially only reflected light, can be induced to emit a mixed color spectrum under direct UV excitation, followed by a different (possibly also mixed) spectrum after the UV has been removed.

[0068] As with previous embodiments described herein, multiple UV induced color-changing effects can be integrated onto a substrate in close proximity to provide not only enhanced features, but interesting ones as well. Patterning of multiple such features in, for instance, concentric circles, stripes, checks and squares, etc. can provide unique indicia which can be used to identify or authenticate the object which includes the indicia.

[0069] The embodiment of Figure 14 uses multiple layers of material to achieve the desired effect. A first layer of fluorescent particles with time constant of emission τ₁, is applied to a substrate, followed by a second layer of fluorescent material (e.g. a phosphorescent material) with time constant of emission τ₂, not equal to τ₁. Additional layers can be applied exceeding two. The pigments can be nearly invisible in ambient light enabling them to be applied to substrate with other pigments without detracting from them. When exposed to UV, all layers will begin to emit their characteristic emission spectra resulting in a mixed spectrum being presented to an observer. Upon cessation of the UV, the returned color spectrum will change as one by one the shorter emission time-constant materials cease to emit light. This embodiment provides for a system that changes from one to at least one other color as it is removed from UV illumination.

[0070] The embodiment of Figure 15 integrates multiple spectral functionalities into a layered particle system to achieve a dynamic response to UV. It uses two or more fluorescent layers with time constants τ₁ and τ₂, where τ₁ >>τ₂. Upon UV excitation, all of the fluorescent materials emit light of their characteristic wavelength bands returning a mixed spectrum to the observer. Upon cessation of UV, the shorter-lived τ₂ (shown green) material fades, while the more phosphorescent τ₁ material (shown red) continues to glow for some period. Thus the color of the substrate appears to shift from yellow (red + green) to red when the material is exposed to and then removed from the source of UV.

[0071] The embodiment of Figure 16 uses an intensity dependent emission spectrum as the feature. A monolithic particle, that responds to focused UV light by emitting a first color spectrum, which changes as the intensity of the excitation is reduced (defocused) is the basis for this embodiment. The particle is integrated onto the substrate using an appropriate carrier. When exposed to focused UV light it emits a characteristic spectrum which includes transitions that require higher intensity to be induced. As the focus of the UV light is moved beyond the substrate surface, and the intensity is reduced, the color spectrum of the emission changes. The particles or layers used in the embodiments of Figure 16 may be created by forming particles or layers which include two (or more) fluorescent chromophores, one of which has a much higher efficiency (in converting received UV into fluorescent emissions) than the other fluorescent chromophore. The higher efficiency fluorescent chromophore will fluoresce much more than the lower efficiency fluorescent chromophore and will shift the resulting output color spectrum toward the color of the higher efficiency fluorescent chromophore when the illumination intensity is low (e.g. a defocused UV light source) and when the illumination intensity is high, the lower efficiency fluorescent chromophore will contribute more of its color to the resulting output color spectrum.

[0072] In the foregoing specification, the inventions have been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the inventions as set forth in the claims which are appended hereto. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.

[0073] The following is a list of thirty-eight clauses which reflect the corresponding claims of the parent application as filed and which form part of the entire disclosure hereof. The relevant subject-matter for which protection is sought is separately defined in the set of claims appended hereto.

1. A pigment particle comprising:

a core having a carrier substance and at least one of a fluorescent material and a phosphorescent material, the core having a substantially spherical shape;

a shell surrounding the core, the shell including a photochromic material which has a first optical property in a first light source and a second optical property in a second light source which includes a set of wavelengths not sufficiently present in the first light source, and wherein the second optical property attenuates an emitted radiation from the at least one of the fluorescent material and the phosphorescent material.

2. A pigment particle as in clause 1 wherein the at least one of the fluorescent material and the phosphorescent material comprises a fluorescent chromophore and wherein the first optical property is substantially transparent and the second light source comprises ultraviolet (UV) wavelengths which are the set of wavelengths and wherein UV excitation from UV in the second light source initially causes the pigment particle to have a first appearance based on fluorescent emissions from the fluorescent chromophore and wherein continued UV excitations causes the pigment particle to have a second appearance based on the second optical property of the photochromic material which attenuates an emitted radiation from the at least one of the fluorescent material and the phosphorescent material.

3. A pigment particle as in clause 2 wherein the first appearance is a first color and the second appearance is a second color which is different than the first color and wherein the photochromic material changes from the first optical property to the second optical property, while under UV illumination from the second light source, over a period of more than one-third second to less than 20 seconds, such that the first color is visible for at least one-third second to as long as 20 seconds.

4. A substrate which includes the pigment particle of clause 1.

5. A substrate as in clause 4, wherein the substrate is currency and wherein the pigment particle is dispersed in ink which has been printed onto the substrate.

6. A pigment particle as in clause 1 further comprising:

a further shell surrounding the shell, the further shell comprising a further fluorescent material which emits radiation in response to UV illumination, and wherein the second optical property of the photochromic material does not attenuate the radiation from the further fluorescent material.

7. An object comprising:

a substrate;

an indicia comprising a first material and a second material; wherein the first material is coupled to the substrate, the first material having a first optical appearance without ultraviolet (UV) excitation and a second optical appearance after receiving UV excitation; and wherein the second material is coupled to the substrate and is capable of providing a color; and wherein the indicia appears to radiate a first color upon initial UV excitation while the first material has the first optical appearance and then appears to radiate a second color upon continued UV excitation as the first material has changed to the second optical appearance.

8. An object as in clause 7 wherein the second material comprises a fluorescent chromophore which emits fluorescent radiation in response to UV illumination.

9. An object as in clause 8 wherein the first material comprises a photochromic material which, when having the second optical appearance, attenuates the fluorescent radiation from the fluorescent chromophore, and wherein the first color is based on the fluorescent radiation from the fluorescent chromophore.

10. An object as in clause 9 wherein the second color is based on reflections from the first material in the second optical appearance and wherein the photochromic material changes from the first optical appearance to the second optical appearance, while under UV excitation, over a period of more than one-third second to less than 20 seconds, such that the first color is visible for at least one-third second to as long as 20 seconds.

11. An object as in clause 9 wherein the second color is based on fluorescent radiation from a third material and wherein the photochromic material changes from the first optical appearance to the second optical appearance, while under UV excitation, over a period of more than one-third second to less than 20 seconds such that the first color is visible for at least one-third second to as long as 20 seconds.

12. An object as in clause 9 wherein the object is currency and the first material is disposed in a first layer and the second material is disposed in a second layer which is attached to the first layer.

13. An object comprising:

a substrate;

an indicia comprising a first material and a second material; wherein the first material is coupled to the substrate, the first material having a first optical appearance without ultraviolet (UV) excitation and a second optical appearance after receiving UV excitation; and wherein the second material is coupled to the substrate and is capable of providing a color; and wherein the indicia appears to radiate a first color at a first intensity upon initial UV excitation while the first material has the first optical appearance and then appears to radiate the first color at a second intensity upon continued UV excitation as the first material has changed to the second optical appearance.

14. An object as in clause 13 wherein the second material comprises a fluorescent chromophore which emits fluorescent radiation in response to UV illumination, and wherein the first material comprises a photochromic material which, when having the second optical appearance, attenuates the fluorescent radiation from the fluorescent chromophore and wherein the first color is based on the fluorescent emission from the fluorescent chromophore.

15. A method to authenticate an object, the method comprising:

exposing a material on the object to a set of wavelengths;

observing radiation of a first color in response to the exposing;

observing radiation of a second color in response to the exposing and after the observing of radiation of the first color, wherein the observing radiation of the first and the second colors is used to determine an authenticity of the object.

16. A method as in clause 15 wherein the material has an initial color prior to the exposing and wherein the material appears to change from the initial color then to the first color and then to the second color in response to the exposing and, after the exposing is stopped, the material reversibly returns to the initial color and wherein the time between changing from the first color to the second color is more than one-third second to less than 20 seconds, such that the first color is visible for at least one-third second to as long as 20 seconds.

17. A method as in clause 16 wherein the material comprises a photochromic material and at least one of a fluorescent material and a phosphorescent material and wherein the object is a currency.

18. A method to authenticate an object having a material, the method comprising:

observing, under ambient light, a reflected color from the material, the material including at least one of a photochromic or saturable absorber which responds to a wavelength which is not sufficiently present in the ambient light;

exposing the material to the wavelength to cause a change in an optical property in the photochromic or saturable absorber;

observing, in response to the change in the optical property, a first color which is different than the reflected color, wherein the observing under ambient light and under the wavelength is used to determine an authenticity of the object.

19. A method as in clause 18 wherein the first color is caused, at least in part, by at least one of a fluorescent material and a phosphorescent material which is part of the material and wherein the material forms an indicia on the object and wherein the change in optical property occurs over a period of time of more than about one-third second to less than about 20 seconds, and wherein after the exposing is stopped, the material, under ambient light, shows the reflected color.

20. A method as in clause 19 wherein the reflected color is produced by a pigment in the material.

21. An object, comprising:

a substrate;

a pigment coupled to the substrate;

an absorption varying material coupled to the pigment and disposed over the pigment; and

a UV florescent material coupled to the absorption varying material and disposed over the absorption varying material.

22. The object of clause 21 wherein the pigment, absorption varying material, and UV florescent material are coupled in layers on the substrate.

23. The object of clause 21 wherein the layers form patterns on the substrate.

24. The object of clause 22 wherein the pigment is layered over the substrate, the absorption varying material is layered over the pigment, and the UV florescent material is layered over the absorption varying material.

25. The object of clause 21 wherein the substrate is currency.

26. The object of clause 21 wherein the pigment is mixed from multiple pigments.

27. The object of clause 21 wherein the absorption varying material is a photochromic substance.

28. The object of clause 21 wherein the photochromic substance is organic.

29. The object of clause 21 wherein the absorption varying material is a saturable absorber.

30. The object of clause 21 wherein the absorption varying material is a saturable reflector.

31. The object of clause 21 wherein the UV florescent material emits visible light upon absorption of UV light.

32. The object of clause 21 wherein the UV florescent material emits infrared light upon absorption of UV light.

33. An object comprising:

a substrate;

an indicia comprising a fluorescent material and a phosphorescent material, the fluorescent material emitting a fluorescent radiation and the phosphorescent material phosphorescing a phosphorescent radiation when under the illumination from the radiation source and continuing to phosphoresce for a period of time after the illumination is stopped.

34. An object as in clause 33 wherein the fluorescent radiation and the phosphorescent radiation create a first color when under the illumination and the phosphorescent radiation, after the illumination is stopped, creates a second color which is different than the first color.

35. An object as in clause 34 wherein the fluorescent material has a first time constant for emission of τ₁ and the phosphorescent material has a second time constant for emission of τ₂ and wherein τ₂ > τ₁ and wherein the illumination comprises ultraviolet (UV) radiation and wherein the indicia changes from the first color to the second color in less than 20 seconds.

36. A method to authenticate an object, the method comprising:

exposing a material with an illumination at a first intensity and observing, in response to the illumination at the first intensity, a first color;

exposing the material with an illumination at second intensity and observing, in response to the illumination at the second intensity, a second color.

37. A method as in clause 36 wherein an indicia on the object comprises the material, and wherein the material comprises a first fluorescent material having a first efficiency and a second fluorescent material having a second efficiency which is significantly less than the first efficiency.

38. A method as in clause 36 wherein the material comprises a first phosphorescent material and a second phosphorescent material.

Claims

1. System or indicia for use as a security or identification feature comprising a combination of materials that is capable of inducing a change of optical appearance of the system or indicia in response to exposure to ultraviolet (UV) light,
wherein said combination of materials includes at least a first material that changes its optical appearance under exposure to UV light and a second material that provides a color under ambient visible light or under exposure to UV light,
and wherein the first and second materials are selected in such a way that exposure to UV light causes a visible spectrum of the system or indicia to dynamically shift over time from a first spectrum to a second spectrum distinct from the first spectrum.

2. System or indicia as defined in claim 1, wherein the first and second materials are selected in such a way that exposure to UV light causes a visible spectrum of the system or indicia to dynamically shift over time from a first spectrum having a first color spectrum to a second spectrum having a second color spectrum distinct from the first color spectrum (Figures 1-6, 8-10, 12-16).

3. System or indicia as defined in claim 1, wherein the first and second material are selected in such a way that exposure to UV light causes a visible spectrum of the system or indicia to dynamically shift over time from a first spectrum having a first color spectrum and a first intensity to a second spectrum having the first color spectrum and a different intensity compared to the first intensity (Figures 7, 9, 11).

4. System or indicia as defined in any one of claims 1 to 3, wherein the first spectrum is visible under ambient visible light and the second spectrum is visible under exposure to UV light (Figures 1, 3, 5, 7, 8, 11).

5. System or indicia as defined in any one of claims 1 to 3, wherein the first spectrum is visible under initial exposure to UV light and the second spectrum is visible under extended/continued exposure to UV light figures 2, 4, 6, 9-12).

6. System or indicia as defined in claim 1 or 2, wherein the first spectrum is visible under exposure to UV light and the second spectrum is visible upon cessation of the exposure to UV light (Figures 13, 14, 15).

7. System or indicia as defined in claim 1 or 2, wherein the first spectrum is visible under exposure to UV light having a first excitation intensity and the second spectrum is visible under exposure to UV light having a second excitation intensity different from the first intensity (Figure 16).

8. System or indicia as defined in claim 2, wherein the first color spectrum is visible under initial exposure to UV light and the second color spectrum is visible under extended/continued exposure to UV light,
wherein the second material comprises a fluorescent or phosphorescent material which emits a visible fluorescent or phosphorescent spectrum in response to exposure to UV light,
and wherein the first material comprises an absorption varying material such as a positive or negative photochromic material, a saturable absorber material, or a reverse saturate absorber material that changes from a first optical appearance to a second optical appearance, while under exposure to UV light, which absorption varying material attenuates the visible fluorescent or phosphorescent spectrum of the second material when having the first or second optical appearance (Figures 2, 4, 6, 9, 10, 12).

9. System or indicia as defined in claim 8, wherein the absorption varying material darkens under exposure to UV light and attenuates, when changing to the second optical appearance, the visible fluorescent or phosphorescent spectrum of the second material, and wherein the first color spectrum is based on or comprises the visible fluorescent or phosphorescent spectrum of the second material (Figures 2, 4, 6, 9, 10, 12).

10. System or indicia as defined in claim 9, further comprising a third material, which third material is a fluorescent or phosphorescent material which emits a visible fluorescent or phosphorescent spectrum in response to exposure to UV light which is distinct from the visible fluorescent or phosphorescent spectrum of the second material,
wherein the visible fluorescent or phosphorescent spectrum of the third material is not affected by exposure of the absorption varying material to UV light,
wherein the first color spectrum is based on additive mixture of the visible fluorescent or phosphorescent spectrums of the second and third materials,
and wherein the second color spectrum is based on the visible fluorescent or phosphorescent spectrum of the third material (Figures 2, 10).

11. System or indicia as defined in claim 8, wherein the absorption varying material fades under exposure to UV light and allows, when changing to the second optical appearance, emission of the visible fluorescent or phosphorescent spectrum of the second material, and wherein the second color spectrum is based on or comprises the visible fluorescent or phosphorescent spectrum of the second material (Figures 2, 4, 6, 10, 12)

12. System or indicia as defined in claim 11, further comprising a third material, which third material is a fluorescent or phosphorescent material which emits a visible fluorescent or phosphorescent spectrum in response to exposure to UV light which is distinct from the visible fluorescent or phosphorescent spectrum of the second material,
wherein the visible fluorescent or phosphorescent spectrum of the third material is not affected by exposure of the absorption varying material to UV light,
wherein the first color spectrum is based on the visible fluorescent or phosphorescent spectrum of the third material,
and wherein the second color spectrum is based on additive mixture of the visible fluorescent or phosphorescent spectrums of the second and third materials (Figure 2, 10).

13. System or indicia as defined in claim 2, wherein the first spectrum is visible under ambient visible light and the second spectrum is visible under exposure to UV light,
wherein the second material comprises a reflective layer which emits a visible reflected spectrum having a rejected color,
and wherein the first material comprises an absorption varying material such as a positive or negative photochromic material, a saturate absorber material, or a reverse saturate absorber material that changes from a first optical appearance to a second optical appearance, while under exposure to UV light, which absorption varying material blocks the visible reflected spectrum of the second material when having the first or second optical appearance (Figures 1, 3, 5,8).

14. System or indicia as defined in claim 13, wherein the absorption varying material darkens under exposure to UV light and blocks the visible reflected spectrum of the second material, and wherein the first color spectrum is based on the visible reflected spectrum of the second material (Figures 1, 3, 5, 8),

15. System or indicia as defined in claim 14, further comprising a third material, which third material is a fluorescent or phosphorescent material which emits a visible fluorescent or phosphorescent spectrum in response to exposure to UV light which is distinct from the visible reflected spectrum of the second material,
wherein the visible fluorescent or phosphorescent spectrum of the third material is not affected by exposure of the absorption varying material to UV light,
wherein, upon initial exposure to UV light, the visible spectrum initially shifts from the first color spectrum to an initial color spectrum that is based on a combination of the visible reflected spectrum of the second material and of the fluorescent or phosphorescent spectrum of the third material,
and wherein, upon extended/continued exposure to UV light, the visible spectrum then shifts from the initial color spectrum to the second color spectrum, which second color spectrum is based on or comprises the visible fluorescent or phosphorescent spectrum of the third material (Figures 1, 3).

16. System or indicia as defined in claim 13, wherein the absorption varying material fades under exposure to UV light and allows reflection of the visible reflected spectrum of the second material, and wherein the first color spectrum is based on the visible reflected spectrum of the second material (Figures 1, 3, 5)

17. System or indicia as defined in claim 16, further comprising a third material, which third material is a fluorescent or phosphorescent material which emits a visible fluorescent or phosphorescent spectrum in response to exposure to UV light which is distinct from the visible reflected spectrum of the second material,
wherein the visible fluorescent or phosphorescent spectrum of the third material is not affected by exposure of the absorption varying material to UV light,
wherein, upon initial exposure to UV light, the visible spectrum initially shifts from the first color spectrum to an initial color spectrum that is based on or comprises the visible fluorescent or phosphorescent spectrum of the third material,
and wherein, upon extended/continued exposure to UV light, the visible spectrum then shifts from the initial color spectrum to the second color spectrum, which second color spectrum is based on a combination of the visible reflected spectrum of the second material and of the visible fluorescent or phosphorescent spectrum of the third material (Figure 1, 3).

18. System or indicia as defined in any one of claims 8 to 17, wherein the absorption varying material changes from the first optical appearance to the second optical appearance, while under exposure to UV light, over a period of time, such that the change from the first color spectrum to the second color spectrum is observable.

19. System or indicia according to any one of claims 8 to 18, wherein the absorption varying material reversibly returns to its first optical appearance after exposure to UV light is stopped

20. System or indicia as defined in claim 3, wherein the first spectrum is visible under ambient visible light and the second spectrum is visible under exposure to UV light,
wherein the second material comprises a reflective layer which emits a visible reflected spectrum having a rejected color,
wherein the first material comprises a broad spectrum absorption varying material that darkens under exposure to UV light and attenuates the intensity of the visible reflected spectrum of the second material,
and wherein the first color spectrum is based on the visible reflected spectrum of the second material (Figure 7).

21. System or indicia as defined in claim 3, wherein the first spectrum is visible under initial exposure to UV light and the second spectrum is visible under extended/continued exposure to UV light,
wherein the second material comprises a fluorescent or phosphorescent material which emits a visible fluorescent or phosphorescent spectrum in response to exposure to UV light,
wherein the first material comprises a broad spectrum absorption varying material that darkens under exposure to UV light and attenuates the intensity of the visible fluorescent or phosphorescent spectrum of the second material,
and wherein the first color spectrum is based on the visible fluorescent or phosphorescent spectrum of the second material (Figures 9, 11).

22. System or indicia as defined in claim 6, wherein the second material comprises a fluorescent material which emits a visible fluorescent spectrum in response to exposure to UV light,
and wherein the first material comprises a phosphorescent material which emits a visible phosphorescent spectrum in response to exposure to UV light and continues to phosphoresce for a period of time after cessation of the exposure to UV light.

23. System or indicia as defined in claim 22, wherein a first color spectrum is produced while under exposure to UV light, which first color spectrum is based on additive mixture of the visible fluorescent and phosphorescent spectrums,
and wherein a second color spectrum is produced after cessation of the exposure to UV light, which second color spectrum is based on the visible phosphorescent spectrum.

24. System or indicia as defined in claim 7, wherein the first excitation intensity is created by focused UV light and the second excitation intensity is created by defocused UV light,
wherein the first and second materials are fluorescent chromophores, one of which having a much higher efficiency in converting received UV light into a visible fluorescent spectrum than the other fluorescent chromophore (Figure 16).

25. System or indicia as defined in any one of the preceding claims wherein the combination of materials is integrated in multiple layers, in heterogeneous pigment particles, or in a mixed particle system.

26. System or indicia as defined in claim 25, wherein the combination of materials is integrated in pigment particles that are mixed with an ink.

27. Ink for printing onto currency or other objects, comprising the system of claim 26 mixed with said ink.

28. Object, especially currency, comprising a substrate and an indicia according to any one of claims 1 to 26.

29. A method to authenticate an object according to claim 28, the method comprising:

- exposing the object to UV light, and

- observing the resulting shift over time of the visible spectrum of the indicia from the first spectrum to the second spectrum.

Drawing